N H 640 autowrap

I bought a N H 640 autowrap on saturday at an auction.  nice clean piece of equipment.  brought it home and greased it up and oiled the chains.  never used one like this before and the auctioneer said no monitor necessary, just start baling.  so i had some hay down and ready when i got home so i decided to try it out.   i realized there was a manual trip lever on it so i figured that would  be an alternative.  i baled until the arrow got into the stop zone and didn't see or hear anything so i stopped and tripped the lever.  it tied real nice, then it kept going without cutting the string and started wrapping again.  figured that was what it is was supposed to do.  then it kept going. so i shut it off when the string arms came back to the right position. cut the string and discharged the bale.  started it back up and it still kept wanting to tie.   after more investigation i realized the slide bar that rotates the knife cutting the string and stopping the dog on the wheel that runs the tying mechanism was not sliding back into place to stop the tie.   i can push it so its not frozen/rusted.   what pushes or holds that in place  after the string arms come back in the baling position ?

Thanks

N H 640 autowrap